As an antenna in the related art, Japanese Unexamined Patent Application Publication No. 2008-99234 (Patent Literature 1) discloses an example of a known antenna used in a communication system disclosed. The antenna disclosed in Patent Literature 1 will now be described below with reference to the accompanying FIGS. 17 to 19. FIG. 17 is a block diagram of a communication system 500 disclosed in Patent Literature 1. FIG. 18 is a perspective view of antennas 520 and 550 used in the communication system 500 illustrated in FIG. 17. FIG. 19 is an equivalent circuit diagram of the antenna 520 illustrated in FIG. 18.
The communication system 500 disclosed in Patent Literature 1 is a system capable of achieving large-capacity transmission by transmitting a high-frequency signal through electric field coupling. More specifically, high-volume data communication can be performed using weak radio waves by applying a communication method, such as a UWB (ultra wide band) communication method, using high frequencies and a wide frequency band to electric field coupling. As illustrated in FIG. 17, the communication system 500 includes a transmission-side electronic apparatus 501 and a receiving-side electronic apparatus 511.
The electronic apparatus 501 includes a transmission circuit unit 502, a resonating unit 504, and a transmission electrode 506. The transmission circuit unit 502 is a circuit for generating a high-frequency signal such as a UWB signal. The transmission electrode 506 emits the high-frequency signal generated by the transmission circuit unit 502 as a radio wave. The resonating unit 504 performs impedance matching between the transmission circuit unit 502 and the transmission electrode 506.
On the other hand, the electronic apparatus 511 includes a receiving circuit unit 512, a resonating unit 514, and a receiving electrode 516. The receiving electrode 516 is coupled to the transmission electrode 506 by electric field coupling and receives the radio wave emitted from the transmission electrode 506. The receiving circuit unit 512 performs demodulation and decoding on the radio wave received by the receiving electrode 516. The resonating unit 514 is a circuit for performing impedance matching between the receiving circuit unit 512 and the receiving electrode 516.
The transmission electrode 506 will be described in detail. As illustrated in FIG. 18, the transmission electrode 506 is a part of the antenna 520. Referring to FIG. 17, the antenna 520 is not illustrated and only the transmission electrode 506 is illustrated. As illustrated in FIG. 18, the antenna 520 includes the transmission electrode 506, a substrate 522, a ground electrode 524, a stub 526, a substrate 528, and via-hole conductors 530 and 532.
The substrate 522 is made of an insulating material. The ground electrode 524 is disposed on the entire undersurface of the substrate 522, and a ground potential is applied to the ground electrode 524. The stub 526 is a linear electrode disposed on the surface of the substrate 522, and has a length approximately half (λ/2) of the wavelength of a high-frequency signal transmitted and received in the communication system 500. The substrate 528 is made of an insulating material, and is disposed on the surface of the substrate 522 so that it partly covers the stub 526. The transmission electrode 506 is a rectangular electrode disposed on the surface of the substrate 528. The via-hole conductor 530 connects the transmission electrode 506 and the stub 526. The via-hole conductor 532 connects the stub 526 and the ground electrode 524. As illustrated in FIG. 19, the via-hole conductor 530 is connected to the stub 526 at a position apart from the via-hole conductor 532 by a quarter (λ/4) of the wavelength of a high-frequency signal transmitted and received in the communication system.
On the other hand, like the transmission electrode 506, the receiving electrode 516 is a part of the antenna 550 illustrated in FIG. 18. As illustrated in FIG. 18, the antenna 550 includes the receiving electrode 516, a substrate 552, a ground electrode 554, a stub 556, a substrate 558, and via-hole conductors 560 and 562. The structure of the antenna 550 is the same as that of the antenna 520, and description thereof can be inferred from the description of antenna 520 provided above.
In the antennas 520 and 550 having the above-described structure, the transmission electrode 506 and the receiving electrode 516 are close to each other so that a predetermined distance (for example, 3 cm) is set between the transmission electrode 506 and the receiving electrode 516. More specifically, the antenna 520 is designed so that a predetermined capacitance is generated between the transmission electrode 506 and the receiving electrode 516 and the input impedance of the antenna 520 and the output impedance (for example 50Ω) of the transmission circuit unit 502 match (that is, impedance matching) when the distance between the transmission electrode 506 and the receiving electrode 516 becomes a predetermined distance. Similarly, the antenna 550 is designed so that a predetermined capacitance is generated between the transmission electrode 506 and the receiving electrode 516 and the output impedance of the antenna 550 and the input impedance of the receiving circuit unit 512 match (that is, impedance matching) when the distance between the transmission electrode 506 and the receiving electrode 516 becomes a predetermined distance. As a result, the reflectivity of a high-frequency signal output from the transmission circuit unit 502 becomes low, and the high-frequency signal is input into the antenna 520. Since the stub 526 has a length approximately half of the wavelength of the high-frequency signal, a standing wave is generated at the stub 526 as illustrated in FIG. 19. A similar phenomenon occurs in the antenna 550 and the receiving circuit unit 512, and the description thereof can be inferred from the above description.
As described previously, the via-hole conductor 530 is connected to the stub 526 at the position apart from the via-hole conductor 532 by a quarter (λ/4) of the wavelength of a high-frequency signal. As illustrated in FIG. 19, this position corresponds to an antinode of a standing wave. That is, the via-hole conductor 530 is connected to the stub 526 at a position at which the largest potential change is obtained. As a result, the change in the potential of the transmission electrode 506 becomes the largest. An electric field having a large amplitude is therefore emitted from the transmission electrode 506 as a radio wave. On the other hand, in the antenna 550, a high-frequency signal flows in a direction opposite to that of a high-frequency signal in the antenna 520. The operation of the antenna 550 is basically the same as that of the antenna 520, and the description thereof can be understood from the description of the operation of antenna 520. In the above-described communication system, the transmission electrode 506 and the receiving electrode 516 are coupled by electric field coupling and the receiving electrode 516 receives the change in the electric field emitted from the transmission electrode 506, so that the transmission of a high-frequency signal is performed.
The communication system 500 disclosed in Patent Literature 1 has a problem in that it has a low degree of design flexibility. More specifically, as illustrated in FIG. 19, a standing wave is generated at the stub 526. This standing wave is generated as a result of the repeated reflection of a high-frequency signal input from the transmission circuit unit 502 into the stub 526 at both ends of the stub 526.
However, when the input-side end portion of the stub 526 and the node of a standing wave exactly match, the input impedance of the stub 526 becomes 0Ω. Accordingly, the impedance matching between a connector 540 connected to the stub 526 and the stub 526 is broken. As a result, a high-frequency signal cannot enter the stub 526. In order to prevent this situation, in the antenna 520, as illustrated in FIG. 19, the input-side end portion of the stub 526 is slightly shifted from the node of a standing wave. More specifically, the connector 540 is connected to the stub 526 so that the input impedance of the stub 526 and the output impedance of the connector 540 match. That is, as illustrated in FIG. 19, the input-side end portion of the stub 526 is placed at a position apart from the point of connection between the stub 526 and the via-hole conductor 532 by a distance slightly shorter than a half of the wavelength of a high-frequency signal. Consequently, the input impedance of the stub 526 and the output impedance of the connector 540 match, the reflectivity of a high-frequency signal becomes low, and the high-frequency signal is input from the connector 540 to the stub 526.